"following the steps above will certainly improve the quality of your YouTube videos, the quest for the perfect "sweet spot" between file size and quality is far from over. Experiment with the settings yourself and discover what works best for you."
Read more at http://www.google.com/search?hl=en&q=Youtube+widescreen+sweet+spot
I have just switched my video camera to 16:9 to try to utilize youtube's going widescreen, but have yet to find the sweet spot when choosing what format to save it to coming out of the editor.
I'm just shooting to miniDV and in AVI 16:9 not HD.
I am using Ulead video studio II for editing which allows a multitude of filetypes to save the projects as.
I haven't found one yet that works best.
I think I managed to get it to widescreen online, but it doesn't fill the box. And looked bad. Most of my videos are under 5 minutes and I think they limit to 100meg.